At birth, the pulmonary circulation changes dramatically. Pulmonary blood flow increases 8-10 fold while pulmonary arterial (PA) pressure declines. If the pulmonary circulation does not make the transition to a high flow, low pressure circuit, then persistent pulmonary hypertension of the newborn (PPHN) results. PPHN is characterized by increased pulmonary vascular tone and reactivity, resulting in severe central hypoxemia that responds incompletely to the administration of supplemental O2 and other vasodilator stimuli. Neither prevention nor cure is currently available. Treatment options are compromised by incomplete understanding of the mechanisms responsible for postnatal adaptation of the pulmonary circulation. While the acute increase in O2 tension is clearly an essential physiologic stimulus for pulmonary vasodilation in the perinatal period, the identity of the O2 sensor remains unknown. Thus, the mechanism whereby the perinatal pulmonary vasculature senses and vasodilates in response to the acute increase in O2 tension remains incompletely understood. Treatment strategies are compromised by the lack of molecular targets for therapy. This proposal seeks to determine whether the ability of the pulmonary vasculature to respond to an acute increase in O2 tension is developmentally regulated and the subcellular mechanisms that allow the perinatal pulmonary circulation to vasodilate in response to an increase in O2 tension. Evidence suggests that potassium (K+) channels in PA smooth muscle cells (SMC) act as the O2 sensor. Work from our laboratory suggests that the calcium-sensitive K+ (KCa) channel plays a central role in mediating the response of the perinatal pulmonary circulation to an acute increase in O2 tension. We propose the working HYPOTHESIS that oxygen activates a KCa channel through intracellular calcium release to cause PA SMC Em hyperpolarization, contributing to perinatal pulmonary vasodilation. The specific aims are to test the following hypotheses: Aim 1. The KCa channel is a major component of PA SMC O2 sensing in the late-gestation fetus and newborn and is developmentally regulated Aim 2. Activation of the PA SMC KCa channel through Ca2+ sparks contributes to perinatal pulmonary vasodilation. To test these hypotheses, we plan experiments using electrophysiology, dynamic calcium imaging, confocal microscopy, molecular biology, immunocytochemistry, and whole animal physiology. The experiments will provide definitive identification of an O2 sensor in the neonatal pulmonary circulation that may be developmentally regulated and elucidate the subcellular processes that lead to its activation, thereby providing a novel molecular target to address neonatal pulmonary vascular disease.